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HIGH PERFORMANCE CONCRETE COMPOSITION & FEATURES

Source: | Updated: Jun 13, 2016

The composition of high performance concrete (HPC) are almost same as those of Conventional Cement Concretes( CCC ). But, because of lower Water Cement Ratio, presence of Pozzolans and chemical admixtures etc., the HPCs usually have many features which distinguish them from CCCs.

From practical considerations, in concrete constructions, apart from the final strength, the rate of development of strength is also very important. The High performance concrete usually contains both pozzolanic and chemical admixtures. Hence, the rate of hydration of cement and the rate of strength development in HPC is quite different from that of conventional cement concrete(CCC).

The proportioning (or mix design) of normal strength concretes is based primarily on the w/c ratio ‘law’ first proposed by Abrams in 1918. For high strength concretes, however, all the components of the concrete mixture are pushed to their limits. Therefore, it is necessary to pay careful attention to all aspects of concrete production, i.e., selection of materials, mix design, handling and placing.

The proportioning (mix design) of High Performance concrete consists of three interrelated steps :

1) Selection of suitable ingredients – cement, supplementary cementing materials ( SCM ),aggregates, water and chemical admixtures.

2) Determination of relative quantities of these materials in order to produce, as economically as possible, a concrete that has the rheological properties, strength and durability.

3) Careful quality control of every phase of the concrete making process.

COMPOSITION OF HPC

The most common composition of high performance concrete as supplementing cementitious materials or mineral admixtures are:

1. Silica Fume

2. Fly Ash

3. GGBFS(Ground granulated blast furnace slag)

1. Silica Fume in HPC

Silica fume is a waste by-product of the production of silicon and silicon alloys. Silica fume is available in different forms, of which the most commonly used now is in a densified form. In developed countries it is already available readily blended with cement.

It is possible to make high strength concrete without silica fume, at compressive strength of upto 98 Mpa. Beyond that strength level however, silica fume becomes essential. With silica fume it is easier to make HPC for strengths between 63-98 Mpa.

2. Fly Ash in HPC

Fly Ash of course, been used extensively in concrete for many years. Fly ash is , unfortunately, much more variable than silica fumes in both their physical and chemical characteristics. Most fly ashes will result in strengths of not more than 70 Mpa. Therefore for higher strengths, silica fume must be used in conjunction with fly ash.

For high strength concrete, fly ash is used at dosage rates of about 15 % of cement content.

3. Ground granulated blast furnace slag (GGBFS) in HPC:

Slags are suitable for use in high strength concrete at dosage rates between 15-30 %. However, for very high strengths, in excess of 98Mpa, it is necessary to use the slag in conjunction with silica fumes.

Key Features of High Performance Concrete (HPC)

  • Compressive strength > 80 MPa ,even upto 800 MPa

  • Water-binder ratio =0.25-0.35 ,therefore very little free water

  • Reduced flocculation of cement grains

  • Wide range of grain sizes

  • Densified cement paste

  • No bleeding – homogeneous mix

  • Less capillary porosity

  • Discontinuous pores

  • Stronger transition zone at the interface between cement paste and aggregate

  • Low free lime content

  • Endogenous shrinkage

  • Powerful confinement of aggregates

  • Little micro-cracking until about 65-70% of fck

  • Smooth fracture surface


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